1. Field of the Invention
The present invention relates to a planar magnetism sensor utilizing a SQUID (Superconducting QUantum Interference Device) of an oxide superconductor, and more specifically to a sensitive planar magnetism sensor utilizing a SQUID of an oxide superconductor.
2. Description of Related Art
A SQUID is a sensitive magnetism sensor utilizing a Josephson junction, which is capable of measuring extremely weak flux less than flux quantum .phi..sub.O (=h/2e, h: Planck's constant, e: charge of an electron) by converting the flux into voltage. The SQUID has a superconducting loop and one or two Josephson junctions which are inserted to the superconducting loop. The SQUID which has one Josephson junction is called a RF-SQUID and the one which has two Josephson junctions is called a DC-SQUID.
When a SQUID is used for a magnetism sensor, the SQUID is usually associated with a superconducting flux transformer in order to increase its sensitivity. The superconducting flux transformer is constituted of two superconducting coils of different sizes connected in parallel, which are called a pickup coil and an input coil. The input coil is the smaller one which has a size close to that of the SQUID and is placed on the SQUID. The pickup coil is the larger one which detects flux and is arranged apart from the SQUID.
The flux detected by the pickup coil is concentrated to the input coil and the SQUID measures the concentrated flux. By this, the substantial sensitivity of the SQUID increases in relative to the self conductance ratio of the pickup coil to the input coil.
FIG. 1 shows a partial view of a conventional magnetism sensor utilizing a planar DC-SQUID, which is disclosed by M. B. Kechen et al. in Appl. Phys. Lett., 44, 736 (1982). In FIG. 1, the magnetism sensor includes a substrate 5, a SQUID 1 formed on the substrate 5 and a superconducting flux transformer of which input coil 3 is arranged on the SQUID 1. The SQUID 1 is mainly constituted of a washer 10 of a square superconducting thin film, which has a square hole 11 at the center. An input coil 3 of the superconducting flux transformer is placed on the washer 10 of the SQUID 1 but isolated from the SQUID 1.
The superconducting thin film of the washer 10 overlaps at a portion 19 and the overlap portion 19 is connected to the under layer through two Josephson junctions (not shown) connected in series. The washer 10 has two tongue portions 14 and 15, in order to connect the SQUID 1 to a signal processor (not shown).
The input coil 3 is formed of a helical rectangular shaped superconducting coil. The input coil 3 is connected to the pickup coil (not shown) of the superconducting flux transformer, which is formed on the substrate 5 far from the SQUID 1. Namely, the SQUID 1 and the pickup coil is arranged on the same plane. The SQUID 1 measures 100 .mu.m square and the pickup coil measures 1 mm square.
Therefore, when a magnetic field is measured by using the above mentioned conventional planar magnetism sensor, flux penetrates not only the pickup coil but also the SQUID. The flux penetrating the Josephson junctions of the SQUID has adverse effect on the tunnel current flowing through the Josephson junction, so that the voltage generated by the SQUID decreases. The drop of the voltage can reduce substantial sensitivity of the magnetism sensor.